The present invention relates generally to internal combustion engines and, more particularly, relates to a method and apparatus for controlling ignition in a two-stroke engine during engine startup.
Two-stroke internal combustion engines are used in a variety of applications including outboard marine engines, snowmobiles, personal watercraft, snow blowers, and weed trimmers. Many modem two-stroke engines are electronically controlled to the extent that they have electronic ignition and possibly electronic fuel injection. These engines exhibit dramatically improved performance characteristics and reduced emissions than engines that have cam operated fueling and ignition systems. For instance, ignition timing of an electronically controlled engine can be controlled, sometimes on a cylinder-by-cylinder and cycle-by-cycle basis, so as to optimize performance and/or emission characteristics for a particular engine. This is in contrast to a cam-controlled engine, in which ignition timing for a particular cylinder is predetermined by the configuration of the cam controlling the ignition event.
Electronically controlled engines typically attempt to trigger ignition in a particular cylinder at a pre-calculated position of that cylinder, typically within a few degrees of top-dead-center (TDC). Ignition timing control is complicated by the fact that engine position is not monitored continuously but, instead, is detected periodically by detecting the rotation of a number (typically 16 to 24) of equally-spaced indicator markers on a flywheel or another rotational component of the engine past a sensor. However, optimal control of ignition timing often requires triggering of an ignition event at an engine rotational position between two adjacent indicator markers. This optimal ignition position cannot be sensed directly. Ignition timing therefore must be extrapolated from the time of detection of the last-detected indicator marker based upon the engine""s current operational state. Specifically, after the ignition triggering marker (i.e., the indicator marker immediately preceding the optimal ignition position) is detected, an ignition delay period is calculated based on an assumption that the average engine speed at the ignition triggering marker will remain constant until the next indicator marker is detected. Average engine speed can be calculated by summing the number of indicator markers detected over a given period of time, multiplying that number by the known marker-to-marker spacing, and dividing the product by the elapsed time. Ignition then is triggered at the end of the calculated ignition delay period. Hence, if ignition in a particular cylinder is to be triggered 7xc2x0 after TDC for that cylinder and the ignition triggering marker for that cylinder is at TDC, the ignition delay period can be calculated by dividing the angular distance between the ignition triggering marker and the desired ignition location (7xc2x0 in this example) by the engine speed calculated upon the passage of the ignition triggering marker past the sensor.
Calculating ignition timing based on a speed-based extrapolation technique does not represent a problem during steady-state engine operation because engine speed fluctuates very little within a particular cycle. It has been discovered, however, that conventional extrapolation techniques can become ineffective for determining ignition timing during engine startup because engine speed can vary dramatically during the first few revolutions due to rather dramatic pressure fluctuations encountered by the rotating engine. Indeed, the instantaneous engine rotational velocity can vary as much as several hundred rpm or more during just 90xc2x0 of engine revolution. Extrapolations based on average engine velocity therefore have low accuracy during startup. This problem is compounded by the fact that, during the first few revolutions of an engine""s operation, the engine""s control system has very little accumulated data on which to calculate an average engine speed. As a result, ignition delay calculations based on speed-based extrapolations can lead to an error in ignition timing that is so large that ignition of the fuel charge in the cylinder is degraded or even prevented. Hence, errors in ignition timing caused by speed fluctuations of an engine during startup can hinder or even prevent engine starting. These effects become more dramatic in engines having only a few cylinders because pressure and speed fluctuations tend to increase at least generally inversely with the number of cylinders in the engine. Hence, while the problem is noticeable in 6 cylinder engines, it becomes more critical in engines having 4 cylinders or less.
The need therefore has arisen to provide a method and apparatus for controlling ignition timing during startup of a two-stroke internal combustion engine while avoiding ignition timing errors caused by engine speed fluctuations.
Pursuant to the invention, a method of starting a two-stroke engine includes driving a rotational component of the engine to rotate, the component having a plurality of spaced indicator markers located thereon, while detecting rotation of indicator markers past a designated location. One of the indicator markers is an ignition triggering marker that designates a position on the component that is acceptable for triggering ignition in a cylinder of the engine. Ignition in the cylinder is triggered upon detecting the ignition triggering marker without taking engine velocity into account. Ignition timing errors that could be introduced due to erroneous assumptions concerning engine velocity therefore are avoided. The method is particularly beneficial in engines having less than six cylinders because those engines exhibit the most dramatic pressure and speed fluctuations during engine startup.
The method preferably additionally comprises monitoring engine velocity and determining when a calculated engine velocity exceeds a threshold engine velocity, and, after determining that the calculated engine velocity exceeds the threshold engine velocity, changing over to an ignition control scheme that takes calculated engine velocity into account when triggering ignition. The threshold engine velocity preferably is one that at least approaches a minimum idle speed of the engine. In the case of a 75 to 135 hp V-4 engine, the threshold velocity preferably is on the order 300 rpm to 400 rpm. The ignition control scheme implemented after switch-over preferably is one which, upon detection and identification of an ignition triggering marker (which is not necessarily the same triggering marker which was used prior to the switch-over, but which is calculated and can be any of the indicator markers provided the desired ignition event is to occur after the triggering marker passes but before the passage of the second indicator marker thereafter), (1) calculates an engine rotational velocity at the calculated ignition triggering marker, (2) calculates an ignition delay period based on the calculated engine velocity at the calculated ignition triggering marker, and (3) triggers ignition upon expiration of the calculated ignition delay period.
In accordance with another aspect of the invention, a two-stroke engine with improved start capability includes at least one cylinder, a computer, and a starter which, when actuated, drives a rotational component of the engine to rotate. The rotational component has a plurality of indicator markers thereon, one of which designates a position on the component that is acceptable for triggering ignition in a cylinder of the engine. The engine further comprises a monitor which monitors rotation of the rotational component, an electrically powered device which, when energized, triggers ignition in the cylinder, and a computer which is coupled to the monitor and to the powered device. The computer is operable, in conjunction with the monitor and the powered device, to detect rotation of the ignition triggering marker past a designated location, and to trigger ignition in the cylinder upon detecting the ignition triggering marker without taking engine velocity into account.
The engine may be a multi-cylinder engine, in which case the rotating component preferably is provided with a plurality of ignition triggering markers on the rotational component, each of which designates a position on the component that is acceptable for triggering ignition in a respective cylinder. In this case, the computer is further operable to repeat the detecting and triggering operations for each of the cylinders during startup.
Preferably, the computer is further operable to (1) calculate engine velocity, (2) determine when the calculated engine velocity exceeds a threshold engine velocity, and (3) after determining that the calculated engine velocity exceeds the threshold engine velocity, change over to an ignition control scheme that takes calculated engine velocity into account. If the optimum rotational position of the rotational component for triggering ignition in the cylinder is located between a calculated triggering marker and the second downstream indicator marker in the direction of component rotation, the ignition control scheme preferably is one which, for each cylinder, and upon detection of the calculated ignition triggering marker, (1) calculates an engine rotational velocity at the calculated ignition triggering marker, (2) determines an ignition delay period based on the calculated engine velocity, and (3) triggers ignition upon expiration of the calculated ignition delay period.
These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.